Saw this had a lot of views, so I compiled every response I've gotten for easier reading. Good luck with your battles, folks ^^


Earthbender:
(Response credit to Scathreoite)
doesn't really matter how often you take out the wisps I don't think? I mostly just ignore them entirely.
The trick with the boss is, a few turns before the boss is max mana (or just for the whole fight?) Tank In Slot 3.
For whatever reason, my earth tank can just shrug off their lasso+max mana move combo where it straight OHKOs my air attacker. Once you have survived the lasso nonsense down, it isn't nearly as hard
(Response credit to Eternalsilver)
The first turn, I switched my tank (water) into the third slot (make sure that you're switching the correct pets - once I accidentally switched my first and second slot after being so used to doing that with golems. As a result, my air tank perished), then used water and airs ability five. Continue with normal attacks. When the Earthbender is around halfway mana, cast waters second ability on him/herself. A turn before the boss is at full mana, use Divine Balance from light to catch your tank (if they die). When your tank has the little tornado icon, use fairy light to heal any missing HP. The minions will be summoned, but I would advise to just ignore them. The second time through, do the same thing, but since you will not be able to cast divine balance (due to the cooldown), cast lights ability five when the boss mana is around halfway.
During all this, use razor wind from air when possible, and once Earthbenders health is ~50%, use decimate. Finish the boss off, then turn to the fragmented energy.


Voidmancer:
(Response credit to Scathreoite)
Avoid killing the wispybois for a good bit, since them dying boosts the boss, but remember that after a while they're also going to get quite strong.
Seems to be fine if you just take them down over the natural course of the battle via r5 razor wind (or presumably fire attacks) since it takes a while for them to go down that way, meaning it's a reasonable balance between their strength and the boss getting power from them?
if air pet, when you get the boss closish to 50% hp, move5 from air pet (I havn't fought them with a fire pet so no idea there). Torrential blow to increase crit chance, razor wind for mass bleeding debuff, and decimate to take out a huge chunk of hp
(Response credit to Eternalsilver)
At the beginning of the battle, all party pets gain a debuff, mark of corruption, which will drain mana, increasing every 20 turns. The Voidmancer has various buffs, including Empowering Corruption, which increases physical/magical damage every 10 turns, Void Summon, which summons voidlings into the battle every possible turn (killing these voidlings increase the Voidmancers damage, so it is wise to leave them be), and Void Barrier, a buff that weakens over time.
First, I use ability five on both water and air (saving lights ability five for when it is needed most). This lets water gain enough mana for Ice Sphere for when the Voidmancer reaches full mana, and it lets air do extra damage.
The voidlings will be summoned, and they have a buff that increases their physical/magical damage every turn - these can potentially murder your party. Continue with regular attacks until air has enough for Torrential Blow - use this, and continue building up mana for airs ability two, Razor Wind. Use fairy light for healing if needed. Once the Voidmancer reaches full mana, use Ice Sphere, which will absorb most of the damage. Lights Restoration/fairy light can heal any damage done.
Continue with regular attacks, dealing as much damage as you can with your air. When the Voidmancer reaches full mana again, use divine balance and block on the pets who don't have Ice Sphere/Ice Barrier. Use healing items/lights abilities if necessary. If your air has enough mana, use decimate, which will hopefully lower the Voidmancers health to around 20% (whereas previously it was ~50%).
Cast (if possible) Ice Sphere, and use lights ability five, as this is when the Voidlings crits can cripple your team. Try to weaken the Voidmancer's remaining HP, and if your pets don't have defenses when Voidmancer reaches full mana, block (this is advised to do in the later parts of the battle, or all parts if you want to be extra-cautious and have the spare mana). Once the Voidmancer is nicely buried, turn to her minions - and beware! I have died multiple times by these gremlin excuses of a creature, and they are a force to be reckoned with. Be cautious, play defensive, and youll be fine (hopefully). Focus on one at a time, and they'll be buried beside their master in no time.

Nynomax:
(Response credit to Scathreoite)
just wail on him until he's 90mana, block on that turn and the next turn, continue wailing on him
if he summons a specter, take it out asap (block if it gets to full mana, while also still remembeing to block at nyno's mana points) because uh, cant really do much to him while the specter is there
and once it's out, just. keep punching him?
Honsetly he's like. the easiest of the three for me? I lost to him once and then I've won every round with him since then.
Mostly it comes down to making use of air move5 at an opportune moment to get supercharged decimate at a good time
Other move5s also handy
(Response credit to Eternalsilver)
Like in the battle with the Voidmancer of Duvhal, I used water and airs ability five at the very start of the battle. The debuffs for party pets are the same as in the battle with the Voidmancer, as well as the buffs (for Nynomax). Use regular attacks until air has enough for razor wind. Use waters second ability on itself, and block on the other two pets right before Nynomax reaches full mana. Once at full mana, your party will gain a debuff that deals magical damage - use fairy light to heal any missing HP. Rinse and repeat this until the boss buff has a small 3 beside it. If the boss is already murdered, then congratulations! If not, the boss will summon a minion you have to murder before doing any damage to the actual boss. While killing the minion, be sure to pay attention to the boss, too - the debuffs can easily kill a party member while youre distracted.

Adding to this - whoever has an answer pls ping me

Should you ever kill/target the voidlings?

Also, what is your expertise on your pets, as well as equipment?

Thank you. Did you ping Hiraeth, by any chance?

Thank you. Do you possibly know strategies for any others?

I'd love to hear it when you have another chance ^^